CBQE-1512: swap rebalance test setup refactor

Change-Id: I3df5ba456356a2192e04bb87dfa99d7b146b8aae
Reviewed-on: http://review.couchbase.org/28298
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>